Rockstar Games confirmed the leak

A letter confirming the leak was published on the official Twitter account of Rockstar Games.

According to him:

An unidentified third party accessed and downloaded company data, including recordings of early gameplay of the next Grand Theft Auto;
▫️the leak will not cause disruption of the company’s servers and will not have a long-term effect on the development of current projects;
▫️work on the new part of Grand Theft Auto will go according to plan;
▫️ the game will be presented when it is ready.